1. Field of the Invention
The invention relates to the production of prints, especially multicolor prints or offset printing forms, by means of thermal transfer, using a narrow tape as a transfer film. More particularly, the invention relates to a method and apparatus for optimizing the imaging quality of a thermal transfer print of this type.
2. Discussion of the Prior Art
The thermal transfer method has been known in principle for a long time. A substrate, which may be the final substrate or an intermediate carrier, is brought into contact with a colored layer which is applied to a carrier and transfers this colored layer to the substrate, dot by dot and in accordance with an image, by means of the action of heat.
By means of different-colored films, a number of colors can also be applied one after another, and a colored print can thus be produced. If the substrate is an intermediate carrier, the finished multicolor image is then transferred to the target substrate in a further step. Furthermore, a printing form can also be coated in accordance with an image by means of a suitable polymer. If, for example, the base of the printing form is hydrophilic and hence does not accept ink, the image-carrying parts are transferred to this printing form by thermal transfer as a positive and are then hydrophobic, that is to say they accept ink.
Coating a substrate in this way, in particular by means of a laser, is disclosed by German reference DE 44 30 555 C1. This document describes a method and an apparatus by means of which a printing form can be produced simply and in a manner which can be integrated into the printing machine, in particular on a seamless printing-form cylinder with a smooth surface, without the gases which arc produced during the laser imaging operation noticeably interfering with the transfer of material from the thermal transfer film, that is to say the imaging quality.
Here, a tape-like transfer film with a tape width which is only a fraction of the substrate width is guided through between the substrate and the imaging unit, in the immediate vicinity of the substrate surface, by means of the tape transport mechanism. The tape transport mechanism, together with the imaging unit and electronically or mechanically coupled, is fitted to a traversing unit, so that the transfer film can be moved over the substrate width uniformly with the movement of the imaging unit. Together with the laser-induced thermal imaging unit, which is controlled in a known way by means of a control unit in accordance with an image to be transferred and, for each image point, introduces heat into the thermal transfer film, and thus performs a dot-by-dot transfer of the ink-accepting coating of the transfer tape, it is thus possible for the complete substrate, in particular the complete seamless printing-form cylinder, to be imaged all round.
It has transpired that transferring the thermal transfer material in the laser-induced thermal transfer process gives particularly good imaging results as a result of the transfer film rolling synchronously on the cylindrical substrate surface. This can be attributed in particular to the distance between the transfer tape and substrate then being a minimum.
As a result of this small distance, a low relative speed close to zero between the transfer film and the substrate surface leads to adhesion of the transfer tape to the substrate. This is desirable, but necessitates truly exact synchronization, since any slight relative speed which may occur leads to "smearing" of the thermal transfer material on the substrate surface. The laser-induced thermal transfer process leads to the transfer film adhering temporarily to the substrate. If, in the event of a positive relative speed, the force causing the tape to advance is greater than the adhesion, then the bond will be broken and the transfer material will be transferred only to a partial extent and with smearing. As a result of the inherent elasticity of the transfer tape, adhesion alternates with sliding; the so-called "stick--slip" effect occurs: it is therefore essential for the speed of the transfer film and substrate surface to be exactly the same.